The allocation of radio resources to radio bearers employed in a wireless communications network is of particular relevance in packet switched networks.
Wideband Code Division Multiple Access (WCDMA) radio access networks (RAN), for example, comprise a dedicated packet scheduling (PS) module as part of a radio resource management (RRM). This module includes a packet scheduling, algorithm which is responsible for allocating radio resources to the radio bearers.
The ‘packet scheduling’ algorithm calls at regular intervals referred to as capacity grant period-an algorithm for modifying the current bitrate. This ‘bitrate modification’ algorithm is designed to maximize the utilization of the network while at the same time maintaining its stability.
FIG. 1 is a flow chart illustrating a possible implementation of a power-based ‘bitrate modification’ algorithm.
As a first step, the ‘bitrate modification’ algorithm is activated at the beginning of a new capacity grant period. The current total power Pcurrent used by the radio bearers is provided as a parameter to the algorithm. The algorithm then compares the current total power level Pcurrent with a predetermined admission threshold Pthreshold minus a predetermined offset.
In case the current total power Pcurrent is below the predetermined admission threshold Pthreshold minus the predetermined offset, an ‘increase load’ algorithm is called by the ‘bitrate modification’ algorithm. The activated ‘increase load’ algorithm then attempts to increase the current load by running through a request queue in which all packet bearers desiring a higher bitrate have placed a request. The ‘increase load’ algorithm uses some granting criterion to determine whether or not to grant the requested bitrates. When the ‘increase load’ algorithm has completed its task, the ‘bitrate modification’ algorithm is terminated.
In case the current total power Pcurrent exceeds the predetermined admission threshold Pthreshold minus the predetermined offset, the ‘bitrate modification’ algorithm checks whether the current total power Pcurrent moreover exceeds the predetermined admission threshold Pthreshold by itself.
In case it is determined that the current total power Pcurrent is below the predetermined admission threshold Pthreshold, the available resources are supposed to be utilized optimally in the radio access network, and the ‘bitrate modification’ algorithm is terminated.
If it is determined in contrast that the current total power Pcurrent exceeds the predetermined admission threshold Pthreshold, a ‘decrease load’ algorithm is called. The activated ‘decrease load’ algorithm attempts to decrease the current load by decreasing the bitrates of the packet bearers until a criterion similar to the granting criterion is satisfied. When the ‘decrease load’ algorithm has completed its task, the ‘bitrate modification’ algorithm is terminated.
In a power-based packet-scheduling algorithm the granting criterion is based on estimates of the total received wideband power and the total transmitted wideband power at a base transceiver station (BTS).
The principle of any granting criterion is to estimate the impact of the proposed change (increase or decrease) on the system in order to enable the use of as much resources as possible without risking instability. Current algorithms take two approaches.
In a first, simple approach, the-power level after the change is estimated based on the current load and the desired changes. The algorithms then grants capacity to the requesting radio bearers if the estimated power level is below a predetermined threshold. However, the uncertainty of such an estimate can be quite high, especially if a lot of high bitrate packet bearers are served in a cell. Especially non-realtime (NRT) services such as web-browsing have a bursty behavior, and when they go from active to inactive or inactive to active this has a great impact on the actual received or transmitted power level. Assuming that the served NRT bearers are by random inactive at the time of making a decision whether to grant resources to another bearer, the system is thus likely to become instable if some of the NRT bearers turn active right after the admission of additional radio resources to the radio bearers. The result will be a low quality of the transmissions and dropped calls. One way to avoid such situations could be to set the admission threshold to a low value, but this would mean that capacity is wasted in the average case.
In a second approach, the potential power increase for the case that all inactive NRT radio bearers become active is estimated in addition to the power level after the desired changes. Requested additional radio resources are only granted if the sum of both estimates is below some predetermined power threshold. The allocation of radio resources is thus based in this approach on a consideration of the worst case in which all inactive NRT bearers become active. Thereby, instabilities can be avoided reliably, but if the NRT radio bearers occupy a large proportion of available resources and if the activity factor is low, a lot of capacity is wasted. If the NRT radio bearers occupy e.g. 75% of the available resources and the activity factor is ⅓ active time, the-wasted capacity is 50%., This approach can be assumed to be suitable for 3G networks in the first phase after their introduction, but as soon as high-speed NRT services become popular, it will waste a lot of capacity, since the worst case is very unlikely.